Hey Steve,I don't think it would damage anything... but may not be loud enough. I use a guitar tube head as my practice rig. I'll jam with it once I get it overhauled and new Weber drivers for my cabinets

no problem. actually a 100 watt tube head can get pretty friggin' loud with an efficient cab. The main diff between guitar and bass head like the Sunn above is in the tone control voicing and the addition of reverb, sometimes not even the voicing. Most tube heads, especially old ones are fussy about output impedance. For instance on some older Marshalls you have to pull out a 'doo-hickey' on the back of the chassis and re-insert it in another position to go from 8 to 4 ohms. Do check that out and be sure that you have a good match between the head and the cab 'cause you can fry things unless!